This invention relates to digital data encoding/decoding techniques and, more particularly, to such encoding and decoding techniques wherein successive binary bits of first or second values are represented by the separation between succeeding transitions, these transitions having predetermined minimum and maximum separations. The encoded data is particularly applicable for direct recording on a record medium, such as a magnetic tape, a rotary disc, and the like.
Record media and different types of data transmission channels exhibit characteristics such that so-called raw digital data is not easily recorded/reproduced or transmitted/received with sufficient fidelity. To avoid distortion and loss of information, various encoding techniques have been proposed whereby the usual binary "1"s and "0"s are converted to suitably coded form which is more accurately recorded or transmitted. One example of such an encoding technique converts an m-bit data word, formed of m binary bits, into a n-bit data word, as disclosed in copending application Ser. No. 201,781. Another encoding technique is known as a "three position modulation" encoder, whereby digital data is encoded in a so-called look-ahead code by which the density of the recorded data is increased.
In the three position modulation encoder, succeeding binary "1"s are separated from each other by at least two binary "0"s. By reason of this separation, the minimum interval between succeeding transitions is equal to three bit cell intervals, wherein a bit cell is an interval, or duration, occupied by a binary bit. That is, if a binary "1" is represented by, for example, a signal voltage that is greater than the signal voltage representing a binary "0", the transitions between "1"s and "0"s are separated by at least 3T, wherein T is equal to the bit cell interval. If the digital signal which is produced by the three position modulation encoding technique is further converted to, for example, NRZI form, then the minimum separation between succeeding transitions, referred to herein as the transition interval, T.sub.min, and the maximum transition interval T.sub.max, are set at T.sub.min =1.5T and T.sub.max =6T.
When digital data is recorded on a magnetic medium, certain constraints must be placed upon the minimum transition interval T.sub.min. That is, where a high recording data density is desired, the minimum transition interval T.sub.min must be of sufficient duration to avoid a possible misinterpretation of succeeding transitions that are spaced too closely to each other. That is, a transition that may be spaced too closely to another may be missed, or skipped, during a signal reproducing operation, thereby distorting the information which can be recovered. Although the minimum transition interval T.sub.min of the aforementioned three position modulation encoder is satisfactory, the maximum transition interval T.sub.max of that encoder is, in many applications, too long. For example, a maximum transition interval T.sub.max of 6T is not favorably disposed for self-clocking. Hence, the synchronous reproduction of data which has been encoded in three position modulation format may not be easily attained. Consequently, the reproduced data may be distorted, and valuable information may be lost.